The moon appears to be far richer in water than previously believed, following the discovery of 'melt inclusions' in rocks brought back from the Apollo mission in 1972.
Analysis of the tiny globules of molten rock, which have turned to a glass-like material trapped within crystals, indicates that the water content of lunar magma is 100 times higher than thought.
Unlike most volcanic deposits, the inclusions are encased in crystals that prevent the escape of water during eruption.
They were found in lunar sample 74220, the famous high-titanium 'orange glass soil'. It's now been analyzed by a state-of-the-art ion microprobe instrument to measure the water content of the inclusions, which were formed during explosive eruptions on the moon around 3.7 billion years ago.
"These samples provide the best window we have on the amount of water in the interior of the moon where the orange glass came from," said science team member James Van Orman of Case Western Reserve University in Cleveland.
The results raise questions about aspects of the 'giant impact theory' of how the moon was created - which predicts that catastrophic degassing during an early collision of Earth with a Mars-sized body would have led to a very low water content in lunar rock.
The team says the study also strengthens the case for returning similar samples from other planetary bodies in the solar system.
"Water plays a critical role in determining the tectonic behavior of planetary surfaces, the melting point of planetary interiors and the location and eruptive style of planetary volcanoes," said Erik Hauri, a geochemist with the Carnegie Institution of Washington and lead author of the study.
"I can conceive of no sample type that would be more important to return to Earth than these volcanic glass samples ejected by explosive volcanism, which have been mapped not only on the moon but throughout the inner solar system."
The study may also provide a new explanation of the water ice found in polar craters by several recent NASA missions. It's previously been attributed to comet and meteor impacts, but the researchers now believe that some of the ice may have come from water released by the eruption of lunar magmas eons ago.